1. Field of the Invention
The present invention relates to a pixel circuit, and more particularly, to an active matrix organic light emitting diode (AMOLED) pixel circuit that can be implemented with amorphous silicon thin film transistors.
2. Description of the Prior Art
Several conventional active matrix drive schemes that have been used in liquid crystal display (LCDs) and are being investigated for use in AMOLEDs. These schemes include, for example, (1) an area ratio gray scale (ARG) method (M. Kimura, et al, Seiko Epson Corp., Japan, AMLDC2000), (2) a pulse width modulation method (S. Miyaguchi, et al, J. of SID, 7(3), 1999, p. 221–226), and (3) an amplitude modulation method, as used in direct view active matrix liquid crystal displays (AMLCDs).
A display-driving scheme for an array of pixels is dependent on pixel schematic, a computer aided design (CAD) layout and a manner in which control lines are brought out of the array. For example, a prior art AMOLED pixel structure having two NMOS transistors provides a current from a driver transistor that drives an OLED being switched, where the drain of the driver transistor is brought out of the array as a column line. In such a layout, the column line that supplies the current (i.e., the supply line) cannot be scanned in sync with rows lines, but must either be OFF until all the row lines are scanned or must be ON before the row lines are scanned. This aspect, as well as the layout of other control signals (e.g., whether they are brought out as row, column or common lines), dictate possible driving options. Active matrix and OLED technology, together with pixel design, dictate which driving scheme produces a least amount, or an acceptable level of, front-of-screen artifacts. For example, there is a common belief among those skilled in the display art that amorphous silicon (a-Si) thin film transistors (TFTs) are not a suitable technology for driving an OLED display, even though a-Si TFT technology is by choice and sales the mainstream technology used in AMLCDs today. Thus, conventional AMOLED displays are implemented with low temperature polysilicon (LTPS) TFT technology, and to date, no one has implemented an a-Si TFT OLED display. Some of the cited concerns are (1) an insufficient low level of drive current produced by an a-Si TFT (M. Stewart et al, IEEE IEDM, 1998, pp. 871–874; LG, SID 2001), which stems from an inherently low mobility (typically<1 cm2/V/sec and, (2) threshold voltage instabilities (J. Kanicki et al, SID 20th IDRC Proceedings, Sept. 25–28, Palm Beach, Fla., pp. 354–358).
FIG. 1 is a schematic of a conventional TFT-electroluminescent active matrix pixel circuit (T. Brody et al, IEEE TED Vol. 22, No. 9, 1975, pp 739–748). FIG. 2 is a schematic and timing diagram of a conventional matrix array implementation. This same active matrix has also been applied in driving OLEDs, but one problem is that the active matrix is known to suffer from pixel-to-pixel luminance non-uniformity due to variation of a TFT threshold voltage of a driver TFT, e.g., driver Q2, (T. Sasaoka et al, 2001 SID International Symposium Digest (Sony)). Another problem is that gray-scale is related to the drain voltage of the TFT driver Q2, Vd, in a highly non-linear fashion, which makes data driver voltage corrections difficult (S. Tam et al, Proceedings of International Display Workshop 2000, AMD6-3). One of the principal purposes of the active matrix is to provide a frame-period storage in each pixel, where Q1, the pixel's data write TFT, and Cs, the pixel's data storage capacitance, store a pixel voltage as in a conventional a-si TFT LCD display. Unlike a liquid crystal display, where the LC capacitor is a voltage mode light modulator, an electroluminescent phosphor (or OLED) is a current mode light modulator and cannot be used as a voltage mode storage capacitor, thereby incorporating Cs. Cs is incorporated because a current mode light modulator cannot be used as a voltage mode storage capacitor. In addition, because of the current mode light modulator OLED element, driver Q2 provides the necessary driving current.
Several references show implementations and provide discussions of an active matrix having two TFTs per pixel (T. Sasaoka et al, 2001 SID International Symposium Digest (Sony); M. Johnson et al, 2000 International Display Workshop, pp 235–238; S. Tam et al, 1999 International Display Workshop, AMD3-2, M. Kimura et al, Proceedings of International Display Workshop 1999, AMD3-1). In these references, the technology is poly-Si TFT, and the drain of all Q2 TFTs in each pixel are tied together, brought out of the active matrix as a column line, and tied to a DC voltage supply as shown in FIG. 2. The specific Q2 shown in FIG. 2 is shown in each reference listed above, i.e., each of these references include a TFT where all of the Q2 TFTs perform the same role. Because of these limitations and drawbacks, other pixels circuits have evolved, but they rely on three, four or more TFTs per pixel. See for example, (a) U.S. Pat. No. 5,952,789 to Roger Green Stewart and Alfred Ipri, Sep. 14, 1999, (b) U.S. Pat. No. 56,229,506 to Robin Dawson et al, May 8, 2001, and (c) U.S. Pat. No. 6,229,508 to M. Kane, May 8, 2001. However, since it is desirable to maximize fabrication yield, minimizing the number of TFTs per pixel that need to be addressed and minimizing the capacitors per pixel and the number of conductor layer crossovers, which is often proportional to the number of pixel control lines, is given serious priority. In addition the complexity of the driving scheme, and the associated costs for such items as higher performance, larger function drivers and display controllers, will increase for large number of TFTs per pixel that need to be addressed.